1. Field of the Invention
The present invention relates to a semiconductor integrated circuit (IC) including a plurality of circuit blocks for processing signals of different frequencies or levels, and particularly to an improvement for preventing mutual interference between the circuit blocks.
2. Description of the Background Art
Electronic apparatus such as TV tuners or FM/AM tuners need to reproduce an audio signal of a relatively low frequency from a high radio frequency (RF) signal. Consequently such an electronic apparatus often includes a plurality of circuit blocks for processing signals having different frequency bands. For instance, FM tuners used in Japan need to process various signals in a wide frequency range of 20 Hz to 90 MHz including an RF signal of 76 to 90 MHz, an intermediate frequency (IF) signal of 10.7 MHz and an audio signal of 20 to 20,000 Hz.
FIG. 1 is a circuit block diagram showing an example of an FM/AM tuner. An RF signal is received by an antenna 8. An FM front end circuit block 1 which converts a received FM signal to an IF signal includes an RF amplifying circuit 1a, an oscillation circuit 1b and a mixing circuit 1c. The mixing circuit 1c mixes the FM signal amplified by the RF amplifying circuit 1a with an oscillation frequency signal applied from the oscillation circuit 1b and outputs an IF signal. The FM-IF amplifying circuit block 2 limits an amplitude of the IF signal after amplification of the IF signal and then it detects and outputs a stereo composite signal. A multiplex decoder block 4 receives the stereo composite signal through a noise canceller block 3 and demodulates the stereo signal, so that it applies left-channel and right-channel audio signals to respective output terminals 10 constituting a pair. An AM tuner block 5 outputs an audio signal from an AM signal received by the antenna 8.
In recent years, there is an increasing demand for reduction of sizes and enhancement of performance of electronic apparatus. Accordingly, it is desired that all the circuit blocks of the FM/AM tuner in FIG. 1 should be incorporated in one semiconductor chip. However, since the FM front end block 1 processes a high frequency signal of several tens MHz, undesirable radiation is liable to occur, exerting adverse effect on other circuit blocks. On the other hand, since the FM front end block 1 also processes a very low level signal received by the antenna 8, the operation of the block 1 is liable to be unstable due to mutual interference with other circuit blocks and undesirable oscillation might occur in some cases. Accordingly, it is difficult to incorporate the FM front end block 1 in the same semiconductor chip where the other circuit blocks are formed.
FIG. 2 is a schematic sectional view showing a conventional structure for reducing mutual interference between respective circuit blocks in a semiconductor IC. An epitaxial N layer 52 is formed on a P.sup.- semiconductor substrate 51. The epitaxial N layer 52 is covered with an insulating film 55. Right and left islands 58 and 59 where different circuit blocks are formed in the epitaxial N layer 52 are isolated by a pair of P.sup.+ isolation regions 54. A dummy island 53 is provided between the isolation regions 54. Transistors, diodes, capacitors, resistors etc. (not shown) are integrated in each of the circuit block regions 58 and 59. Each of the P.sup.+ isolation regions 54 of the pair is electrically connected with a ground line 57 through an opening 56.
Leakage current from each of the circuit block regions 58 and 59 is absorbed by the corresponding ground line 57 through the P.sup.+ isolation region 54 as shown by the arrows of broken lines in FIG. 2. Consequently, mutual interference due to the leakage current can be prevented between the adjacent circuit blocks 58 and 59. However, if the leakage current is increased, an amount of current flowing in the respective P.sup.+ isolation regions 54 increase, causing an increase of potential in the P.sup.+ isolation regions 54 due to the impedance of the ground lines 57. In such a case, the P.sup.+ isolation regions 54 cannot entirely absorb the leakage current, resulting in a flow of leakage current between the circuit blocks 58 and 59 through the P.sup.- substrate 51. In consequence, the mutual interference between the circuit blocks 58 and 59 cannot be prevented satisfactorily.
In these days, various electronic apparatuses are desired and it is requested to reduce the time for design of semiconductor IC circuit patterns. For example, it is often desired to make changes such as removal, replacement or addition of a specified circuit block in a semiconductor IC. However, the plurality of circuit blocks on the semiconductor chip do not always have the same area and accordingly it is often necessary to redesign the circuit pattern of the whole semiconductor IC due to the removal, replacement, addition or the like of a specified circuit block.